Protective device

ABSTRACT

162,093. British Thomson - Houston Co., Ltd., (General Electric Co.). Jan. 27, 1920. Induction motor relays; protective arrangements.-An induction-type relay comprising an induction disc 5 moving between opposed electromagnets 1, 3 is given an inverse time-characteristic up to a certain value of current above which the time-characteristic is constant irrespective of the current by exciting the winding 6 of the electromagnet 1 by a resultant voltage derived from a transformer 10 excited by the primary winding 11 from the current transformer in the protected line, the resultant voltage being a measure of the primary current up to a predetermined amount and thereafter constant. As shown, the winding 6 is excited by the transformer winding 8 through the adjustable resistance 15 across which an opposing voltage is impressed derived from the connection of the resistance 15 in the adjustable primary circuit 11 of the transformer. Up to a value of current sufficient to saturate the transformer 14. the current in the operating-winding 6 varies with the load current and, consequently, the time of operation of the relay varies correspondingly. Above that current, the opposing voltage at the resistance 15 maintains the current in the winding 6 and, consequently, the time of operation of the relay constant.

B. H. THOMPSON.

PROTECTIVE DEVICE APPLICATION FILED AUG.28. 191'9.

Patented Apr. 26, 1921.

Fig. 2

Voltage Time Cur-rent. Primer-1 Cur-rent.

Inventor: Ber' j amin H.Thompson,

UNIE

BENJAMIN H. THOMPSON, F SCHENECTADY, NEW YORK, ASSIGNOR T0 GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

PROTECTIVE DEVICE.

Specification of Letters Patent.

Patented Apr. 26, 1921.

Application filed August 28, 1919. Serial No. 320,524.

, tain new and useful Improvements in Prowhich:

tective Devices, of which the following is a specification.

My inventionrelates to protective devices and especially to relays and has for its object to provide an overload relay of the re tarded type'which operates with a constant and definite minimum time delay upon overloads in excess of a predetermined amount.

Overload relays of the retarded type operate in a time'which is a measure of the overload'up to a predetermined amount but upon excessiveoverloads, due to conditions of short circuits, such relays frequently operate, practically instantaneously, regardless of their time setting. This feature is undesirable, particularly upon systems of distribution in which a number of these relays are connected to operate in predetermined time intervals or in a predetermined sequence.

In the relay embodying the features of my invention, I overcome these difliculties in the operating of time limit relays, by providing means whereby such relays operate in a time which is a measure of the overload up to a predetermined amount and upon overloads above the predetermined amount, with a definite minimum time which is constant, independent of the excessive' or overload current. This function is obtained by modifying the operating characteristics ofthe relay by means of a device so connected in the relay circuitthat a resultant potential is impressed upon the relay which varies as the excess current up to a predetermined value or amount and then remains constant independent of the current.

I The novel features which I believe to be characteristic of my invention will be definitely indicated in the claims appended hereto, whilethe features of construction and mode' 'of operation will be understood from the following description taken in connection with the accompanying drawings, 1n

Figure 1is a diagrammatic illustration of a time limit overload relay, of the induction I i type, embodying the principles of my invention, and Figs. Qand 3 show graphically the sists of two cooperating electromagnetic elements 2 and 3 and a retarding element; such as a drag magnet 4 operating on a rotatably mounted induction disk 5 to control the relay contacts, not shown. The electro magnetic elements 2 and 3 comprise suitable actuating windings 6 and 7 respectively, with their operating magnetic circuits. These actuating windings 6 and 7 are connected in the circuits of secondary windings 8 and 9, respectively, of a transformer 10 having a primary winding 11 operatively related to the current transformer 12 connected to the conductor 13, the circuit of which is controlled by the relay 1. The auxiliary transformer 10 is, therefore, connected between the main transformer 12 and the relay 1 and has a magnetic circuit 14 arranged to magnetically saturate upon currents above a predetermined amount in conductor 13 and hence in primary winding 11. As both actuating windings 6 and 7 are energized from the auxiliary transformer 10,

the circuit of one winding, for instance winding 6, is made more highly inductive, in order to produce a suflicientphase displacement between the fluxes set up by windings 6 and 7, to create a rot-ative effect on the disk 5. Due solely to the saturation effect of the auxiliary transformer 10, the potential impressed across the secondary windings 8 and 9 and hence the current which will flow through the actuating windings 6 and T of the relay, increases as the first power of the current in conductor 18 up to a predetermined amount which corresponds to the saturation of transformer 10 and on further increases of current, the potential impressed across the windings 8 and 9 is not constant but increases gradually as the primary current increases. This relation can best be shown in Fig. 3 in which the curve A shows the variation in voltage impressed across the secondary winding 8, forinstance, with the variations of current in the primary winding 11, relying merely on the effect of saturation of the transformer '10. It is observed that the voltage across winding,8

continues to increase gradually even after transformer 10 saturates, due to the fact that it is practically impossible to obtain complete magnetic saturation. The effect of the saturation of transformer 10 alone is also shown in Fig. 2 by the curve B which gives the time current characteristics of the relay. This curve, upon currents above a value necessary to cause saturation, has a gradually sloping long, tail portion 0. in relays of this type, the minimum time setting for the relay is not constant and upon extremely heavy overloads such relays will operate practically instantaneously.

According to my invention, it provide means for so modifying the operating characteristic that the relay operates with a constant and definite minimum time of operation on overloads or current values above a predetermined amount or-an amount necessary to saturate transformer 10. Instead of utilizing the potential which is impressed on the secondary winding 8 to furnish the energizing current for the actuating winding 6, I utilize a resultant potential which varies as the primary current up to a predetermined amount and then remains constant independent of further increases in primary currents. For obtaining this resultant potential, I provide a current limiting means, such as a resistance element 15, which is in the circuit of primary winding 11 of auxiliary transformer 10 and acres which a potential drop occurs which varies directly as the current in the primary winding 11. The relation of the potential across element 15, for variations in primary current, is best shown by curve C of Fig. 3. The circuit of the secondary winding 8 is then so related to the circuit including resistance element 15 that the potential impressed on the secondary winding 8 and the potential across the resistance element oppose or buck each other to give a resu ant potential for energizing the actua winding 6 as best shown by curve D of Fig. 3 which is a resultant of curves A and t It will be observed that the resultant -cage impressed across relay winding 6 rises with the current until a current occurs sufficient to magnetically saturate transformer 10 when the resultant potential becon s constant independent of the current in primary winding 11. The modifying effect of the current limiting means or r ance 15 upon the time current characterist. of the relay is best shown in Fig. 2 by the curve E. This curve, it will be obserihas a substantially horizontal or flat tail portion instead of the gradually sloping tail portion of curve B which gives the re lay a constant and definite minimum time of operation upon overloads in excess of a predetermined amount.

As is well known, the number of turns in circuit of the primary winding 11 of the auxiliary transformer 10 is adjustable, the primary winding 11 having taps to vary the pick-up current for the relay, that is the current required to start the relay into operation, and in order to proportionally vary the opposing or bucking potential, the resistance element 15 is also adjustable or has a similar number of taps as the primary winding 11.

In accordance with the provisions of the patent statutes, 1 have described the prin ciple of operation of my invention, together with the apparatus which I now consider to represent the bestembodiment thereof; but I desire to have it understood that the apparatus shown is orny illustrative and that the invention can be carried out by other means.

hat I claim as new and desire to secure by Letters Patent of the United States, is

1. In combination a transformer having a magnetic circuit adapted to saturate on currents above a predetermined amount, a primary and a secondary circuit, for said transformer, means in said primary circuit so related to said secondary circuit to produce a resultant voltage which is a measure of the primary current up to a predetermined amount and is then constant independent of the amount of current, an overload relay, and an actuating winding for said relay energized in response to said resultant voltage.

2. In combination a transformer having a magnetic circuit adapted to saturate on currents above a predetermined amount, a current limiting means in the primary circuit of said. transformer having a voltage drop proportional to the current in said circuit, a secondary circuit so related to said current limiting means that the voltage impressed across said means opposes the voltage iii'iprcssed across the secondary winding of said transformer. and an electroresponsive Ye having an actuating winding enerin response to said resultant voltage. A. relay comprising an actuating winding responsive to overload current, means for im pressing on said winding a potential which varies as the overload current up to a predetermined amount which is constant upon overloads above said predetermined amount, incl ding; a transformer having a s-condary circuit supplying said actuating winding, and means for opposing the potential induced in said secondary circuit by a potential which varies as the current insaid. primary winding.

A relay comprisin operating means responsive to overload current, a transformer controlling said operating means, and means associated with said transformer for impressing potential on said operatmg means which varies as the overload current up to a predetermined amount and is then constant independent of the overload current.

5. In an overload relay, an operating member, electroresponsive means for controlling the movement of said operating member, and means for modifying the characteristics of said electroresponsive means to move said operating member in a time inversely proportional to the overload up to a predetermined amount and in a constant minimum time on overloads above said predetermined amount.

6. In an overload relay an operating member, electro-responsive means for controlling the movement of said operating member, and means for modifying the char acteristics of said electro-responsive means comprising a transformer having a magnetic circuit adapted to saturate on predetermined overloads, a resistance in the primary circuit of said transformer, and a secondary circuit for controlling said operating means so related to said resistance to receive a potential which is a resultant of the potential drop across said resistance and the potential induced in said current.

7. In an overload relay, an operating member, electro-magnetic means controlling said member, a transformer having a primary circuit, a secondary circuit therefor controlling said electromagnetic means and adapted to receive a current proportional to said primary current up to currents of a predetermined amount, and means associated with said primary and secondary circuits to maintain the current in said secondary circuit constant on primary currents above said predetermined amount.

In witness whereof I have hereunto set my hand this 27th day of August, 1919.

BENJAMIN H. THOIWPSON. 

